This invention relates generally to the field of thin film processing through the use of a plasma. Specifically, the invention relates to the occurrence of arcs and the reaction to these arcs such as exist in the initial conditioning of a target material for such processing. It discloses designs which are particularly suited to direct current (DC) operations especially those which involve the application of a switch-mode power supply.
The field of thin film plasma processing is well known. In this field, a power supply creates an AC or DC electric potential which causes the existence of a distributed plasma. The plasma then acts upon some area of material so as to cause processing of an item or substrate. This processing may be deposition, etching, or some other type of processing. Depending on the particular application being pursued, the materials, substrates and even the nature of the processing can vary greatly.
One of the challenges that exists for precise processing is the need to establish a target material in a relatively homogeneous state. This is important because when anomalies exist, areas of low impedance can be created and the plasma may begin to discharge or arc into such areas. Unlike situations in which a cathodic arc is desirable (an undistributed plasma), in such plasma processing applications (defined here as "non arc-based" plasma processing), the existence of arcs can be undesirable. The arcs can negatively impact the desired processing; they can create areas where precise control is not possible. For these and other reasons it has become customary--and often preferred--to use power supplies having very rapid process control capabilities and having very precise energy control. In DC applications this is often achieved through the utilization of switch-mode power supplies. These power supplies act to create DC power which is derived from a high frequency source. This not only affords the opportunity for rapid reaction to plasma conditions but it also allows for very low energy storage so as to minimize the effects caused by arc occurrences within the plasma.
An important aspect in many plasma processing applications is that of conditioning the target or material so that it may be used to achieve more precise processing. Not only are solid targets periodically replaced as they wear down but also they can be removed to allow different processing to occur. In each of these instances, the replaced target can tend to exhibit a tendency to arc as surface anomalies are eliminated. These anomalies may consist of unclean areas (such as fingerprints and the like) or topographic anomalies (such as scratches and the like). Regardless of their source it is not uncommon that target materials need to be conditioned before production line processing can occur.
In applications where not particularly precise processing is required, this conditioning is merely a nuisance. For instance, in the manufacture of products utilizing high energy storage power supplies, it is not atypical to merely throw away the first few products until a new target is properly conditioned. This can take a few seconds and may not be that expensive. Unfortunately in applications where precise plasma processing is desired such as through the utilization of low energy storage power supplies, this conditioning can take thousands of times longer and can result in the disposal of thousands of times more products due to imperfect processing. For instance in the coating of compact disks, rather than merely disposing of a few disks taking a few seconds, when low energy supplies are used, it can be necessary to dispose of many disks over the course of half an hour's run just to condition the target material. Obviously this is undesirable. Other applications can be more extreme as even a single semiconductor wafer may cost almost one hundred thousand dollars. Conditioning in these applications can also take a full day.
Unfortunately, until the present invention no system existed which both achieved quick and efficient conditioning of a target while also achieving the enhanced processing accuracies possible with a low energy storage power supply. Perhaps surprising in this regard is the fact that although there has been a recognized need to achieve both these criteria, this need has remained unsatisfied until the present invention even though the implementing arts and elements had long been available.
This is perhaps due to the fact that those skilled in the art of thin film and non arc-based plasma processing did not fully appreciate the nature of the problem and so were not able to achieve the solutions of the present invention. This also may have been due to the fact that those skilled in the art actually accepted directions which were away from the teaching of the present invention. One of these accepted directions was fueled by the perception that particularly delicate target handling and/or the a need to dispose of a large number of initial product runs was merely a requirement for accurate processing. Although discussed earlier in reference to low energy storage power supplies, even this realization was not fully appreciated. Perhaps also showing the degree to which those skilled in the art accepted and were directed toward handling solutions is also evidenced by the fact that until the present invention it appears that no efforts were made to achieve proper conditioning through a variation of the application of power to the plasma. Thus, until the present invention it simply was not possible to achieve both practical conditioning of a target as well as precise processing through the utilization of a low energy storage power supply such as a switch-mode power supply.